Step attenuators are commonly known and are used in a variety of electronic applications. Exemplary is the attenuator disclosed in U.S. Pat. No. 4,654,610 to DaSilva, which has been widely used in electronic test and measurement equipment.
Step attenuators rely on switches to selectably couple one or more attenuator pads (also known as "sections") into the circuit, thereby defining several different possible attenuation paths. In RF attenuators, the switches are generally activated by control signals which may be toggled by a computer or other control device. (Manually operated switches, such as rotary switches, are generally unsuitable for RF applications.) Among the switches commonly used in step attenuators are electromechanical RF relays and PIN diode switches. Electromechanical relays afford the advantages of high power handling capability, DC coupling, and isolation of the control signal from the signal being switched. PIN diodes offer lower cost and faster switching speed.
Gallium arsenide field effect transistor (GaAsFET) switches have been used in some step attenuator applications, but are generally considered disadvantageous due to their relatively high insertion loss (on the order of 0.8 dB at 1 GHz, as compared to about 0.2 dB for PIN diodes and electromechanical relays). GaAsFET switches do, however, offer superior "off-state" isolation, and are usable at higher frequencies than PIN diodes and relays.
Prior art step attenuators generally follow one of two basic topologies. The first, shown in FIG. 1, is termed a "series" attenuator and is characterized by a plurality of attenuator sections, each of which can be serially interconnected in the circuit, or alternately bypassed, by a corresponding pair of single pole, double throw (SPDT) switches. As can be seen, a series step attenuator with four attenuation pads (thereby defining 15 possible attenuation paths and a through path) requires eight such switches. (It will be recognized that four double-pole, double throw switches can alternatively be used. However, as used herein, "switches" are generally considered to mean single pole devices.)
The second basic step attenuator topology, termed a "ladder" attenuator, is characterized by a chain of serially-coupled attenuator pads, to which the input signal can be introduced at selected points. FIG. 2 shows two such ladder attenuators in series array. The first attenuator, comprised of attenuator pads 10, 12, 14 and 16, is shown as being driven by the input signal at point C, between pads 12 and 14, thereby providing 30 dB of attenuation. More or less attenuation can be provided by introducing the input signal at different points A, B or D in the chain. In this topology, a step attenuator with four attenuation pads (thereby defining four attenuation paths and a through-path) requires five switches.
In accordance with one aspect of the present invention, a new attenuator topology is disclosed that uses a reduced number of switches as compared with conventional designs. In the preferred embodiment, the attenuator topology is arranged to exploit GaAsFETs as switching elements, taking advantage of their high "off-state" isolation, while minimizing the impact of their relatively high insertion loss.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.